RF Directional Couplers are like splitters in that they can feed two lines from one, or combine two signals into one cable or feed. The insertion and coupling loss is different however, when compared to a simple two way with an equal split of one half, or 3 dB. The Directional Coupler will "Tap Off" Some signal to feed a local TV or similar component and send the Majority "Thru" to continue on down the Line to Continue or Repeat this exercize over and over again. This type of Distribution occurrs in "Loop Run" systems that essentially run one cable past each user in a "Loop", Tapping signal to feed each as it passes. This example is used in most Hotels and Motels as the cable runs up or down from floor to floor passing the next room above or below, using a Tap to "tap-off" signal to each room as it passes. In a more Domestic Situation, a coupler is handy to feed a device close to the origin of the signal (like where the Cable TV company attaches to the house outside) and send the majority through a relatively long run to feed other devices at the end. Cable TV Companies do this quite often in "Tapping-Off" signal to feed the Cable Modem. Another Situation would be that one device simply requires more than the other and both would be Accomodated using this Device. They are typically rated in dB as to the Loss incurred from "Input" to "Tap" leg, with the "Thru" Leg suffering comparatively less loss. The Higher the dB Tap value, the lower the "Thru" leg loss.
If you are adding several Directional Coupler Drop Taps to an existing Cable TV or Antenna feed, you may need to compensate for the loss that you are about to introduce. The gain of a Line Amplifier should be selected to zero out the loss that follows it. In other words, if you have fifty feet of cable and a two-way splitter, then a 10 db amplifier will be fine. Conversely, if you have an eight-way splitter and hundreds of feet of cable, you will need an amplifier with a higher gain. Just as in the case of a Pre-Amplifier, however, bigger is not always better.